1. Field of the Invention
This invention relates to a method and apparatus for recording digital video signals on a magnetic tape by means of a plurality of magnetic heads disposed on a rotating drum, and more particularly is directed to compression coding of the digital video signals by transform encoding.
2. Description of the Prior Art
A D1 format component type digital VCR and a D2 format composite type digital VCR have been developed for use by broadcasting stations in digitizing color video signals and recording the digitized signals on a recording medium, such as a magnetic tape. In the D1 format digital VCR, a luminance signal and first and second color difference signals are A/D converted with sampling frequencies of 13.5 MHz and 6.75 MHz, respectively. Thereafter, the signals are suitably processed and then recorded on a tape. Since the ratio of sampling frequencies of the signal components is 4:2:2, this system is usually referred to as the 4:2:2 system.
On the other hand, in the D2 format video digital VCR, a composite video signal is sampled with a signal having a frequency four times higher than the frequency fsc of a color subcarrier signal and then is A/D converted. Thereafter, the resultant signal is suitably processed and then recorded on a magnetic tape.
Since these known D1 and D2 format digital VCRs are designed for professional use, for example in broadcasting stations, the attainment of high picture quality is given top priority in the design and construction of such VCRs, and the weight and size of the apparatus is not overly important.
In these known digital VCRs, the digital color video signal, which results from each sample being A/D converted into, for example, 8 bits, is recorded without being substantially compressed. As an example, when the known D1 format digital VCR A/D converts each sample into 8 bits with the frequencies noted above, the data rate representing the color video signal is approximately 216 Mbps (megabits per second). When the data in the horizontal and vertical blanking intervals are removed, the number of effective picture elements of the luminance signal per horizontal interval and the number of effective picture elements of each color difference signal per horizontal interval becomes 720 and 360, respectively, as shown in FIG. 1A. In the NTSC system (525/60), the number of valid scanning lines for each field is 250, and the valid video data for each field is divided into five segments.
As another example, with respect to the D2 format VCR, in the NTSC system the number of valid picture elements per horizontal period is 768 and the number of valid scanning lines per field is 255, as shown in FIG. 1B. The valid video data for each field is divided into three segments.
In the D1 and D2 formats, various processes such as data element shuffling and error correction encoding are performed. FIGS. 2A and 2B are schematic diagrams showing how each picture data element is distributed to one of a plurality of heads. In the D1 format, as shown in FIG. 2A, four rotating heads, denoted with numerals 0, 1, 2, and 3, are used. On the other hand, in the D2 format, as shown in FIG. 2B, two rotating heads denoted with numerals 0 and 1, are used. Hereinafter, signal paths for the respective heads will be referred to as "channels".
FIGS. 2A and 2B show the channel numbers that are applicable to picture data elements of an even-numbered field of a video signal. For picture data elements of odd-numbered fields, the channel numbers of the even-numbered segments are as shown in the odd-numbered segments of FIGS. 2A and 2B and the channel numbers of the odd-numbered segments are as shown in the even-numbered segments of FIGS. 2A and 2B.
Since digital videotape recording entails handling of large quantities of data, most digital VCRs use a plurality of rotating heads. On occasion, magnetic heads get clogged. When a head gets clogged, all information in the channel corresponding to the head is lost. Therefore, it is customary to distribute recording data to a plurality of rotating heads in such a way that the effect of head clogging is minimized. As shown in FIGS. 2A and 2B, in the D1 and D2 formats, a scheme is used for distributing picture elements among the various channels so that a plurality of spatially adjacent picture elements are not simultaneously lost because of head clogging. In other words, even if a head clogs so that a picture element distributed to that head's channel is lost, the four picture elements which are above, below, and to the right and left of the lost picture element are distributed to other channels and so are not lost. Since the surrounding picture elements distributed to the other channels are properly reproduced, error correction can be accomplished by, for example, substituting the average value of the four surrounding picture elements for the lost element.
In recent years, in addition to the D1 and D2 formats for recording digital video signals, there has been proposed another type of digital VCR using a small rotating drum and a small tape cassette. This type of VCR requires a high performance encoding scheme to compress the large amount of data present in a digital video signal. Two dimensional transform encoding is an example of such a high performance encoding scheme. In two dimensional transform encoding, image data is divided into blocks consisting of, for example, 8.times.8 picture data elements and each block is orthogonally transformed. The transformed elements (referred to as coefficients) are broken down into components from DC to high frequency. Generally, the DC component is large, while the high frequency component is small. By assigning a proper number of bits to each coefficient, the total quantity of bits required for each block can be decreased. Recently the two dimensional discrete cosine transform (DCT) has become a favored transform for purposes of compression coding.
As an example of discrete cosine transformation, let it be assumed that an 8.times.8 block of image data samples is represented as follows:
______________________________________ 139 144 149 153 155 155 155 155 144 151 153 156 159 156 156 156 150 155 160 163 158 156 156 156 159 161 161 162 162 155 155 155 161 161 161 161 160 157 157 157 162 162 161 163 162 157 157 157 162 162 161 161 163 158 158 158 ______________________________________
in which each number in this block represents the magnitude or signal level of the image data sample. When the discrete cosine transform of the 8.times.8 block of image data samples is derived, conversion coefficients c.sub.ij (i represents row number and j represents column number) are produced as follows:
__________________________________________________________________________ 314.91 -0.26 -3.02 -1.30 0.53 -0.42 -0.68 0.33 -5.65 -4.37 -1.56 -0.79 -0.71 -0.02 0.11 -0.33 -2.74 -2.32 -0.39 0.38 0.05 -0.24 -0.14 -0.02 -1.77 -0.48 0.06 0.36 0.22 -0.02 -0.01 0.08 -0.16 -0.21 0.37 0.39 -0.03 -0.17 0.15 0.32 0.44 -0.05 0.41 -0.09 -0.19 0.37 0.26 -0.25 -0.32 -0.09 -0.08 -0.37 -0.12 0.43 0.27 -0.19 0.65 0.39 -0.94 -0.46 0.47 0.30 -0.14 -0.11 __________________________________________________________________________
in which the number representing each conversion coefficient represents the relative power of that conversion coefficient. The conversion coefficient c.sub.00 is referred to as the DC component and represents the mean luminance value of the image block. It is seen that the electric power of the DC component is significantly higher than that of the other components which are known as AC components. As i increases, the frequency of the AC components in the vertical direction increases and as j increases, the frequency of the AC components in the horizontal direction increases. As both i and j increase, the frequency of the AC components in the diagonal direction increases.
The DC component of the conversion coefficients exhibits the largest value and, thus, contains the most information. If the DC component is quantized with a large quantizing step, that is, if it is subjected to coarse quantization, block distortions are produced which appear as noise that is visually detected most readily in the video picture ultimately reproduced from the conversion coefficients, thereby deteriorating the quality of that picture. Consequently, to minimize such visual noise, the DC component of the conversion coefficients, namely c.sub.00, is quantized with a small quantizing step and is represented by a larger number of bits, such as eight or more bits. A lesser number of bits may be used to represent the higher frequency AC components of the conversion coefficients c.sub.ij (where i, j .noteq.0) because higher frequency AC components represent changes in the video information of the n.times.n block and the human eye does not readily detect detail in a rapidly changing image. Consequently, an observer will not sense a loss of detail in that portion of an image which changes from point to point. Therefore, it is not necessary to represent the higher frequency AC components of the conversion coefficients with a large number of bits. This means that a larger quantizing step can be used to quantize the higher frequency AC components of the conversion coefficients. An example of quantizing the conversion coefficients set out above is as follows:
__________________________________________________________________________ 315.00 0.00 -3.00 -1.00 1.00 0.00 -1.00 0.00 -6.00 -4.00 -2.00 -1.00 -1.00 0.00 0.00 0.00 -3.00 -2.00 0.00 0.00 0.00 0.00 0.00 0.00 -2.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1.00 0.00 -1.00 0.00 0.00 0.00 0.00 0.00 __________________________________________________________________________
in which the quantizing is analogous to "rounding off" the conversion coefficients.
In a practical transmission or recording scheme, the quantized conversion coefficients are encoded by variable length coding, such as Huffman coding or run-length coding which provides further data compression. For proper transmission or recording, additional signals, such as synchronizing signals, parity codes, and the like, are added to the variable length coded conversion coefficients.
When a video signal which has been interlace-scanned is encoded within a frame by the DCT scheme, each block is made up of picture elements from corresponding respective portions of an odd field and an even field. In this in-frame encoding scheme, it is not possible to use the conventional channel distribution approach in which data elements from an even field are distributed differently from data elements of an odd field. Moreover, coefficient data which is generated by the two dimensional DCT contains components from DC to high frequency. Thus, the blocks of coefficient data do not have the spatial correlation which is found in the original image data blocks. In addition, if there is an error in only one coefficient in a block of coefficient data, this error affects the entire block. Interpolation for error correction is therefore not possible.